Metal refining process



United States Patent Ofiice 3,230,076 Patented Jan. 18, 1966 of New York No Drawing. Filed Mar. 5, 1963, Ser. No. 262,826 2 Claims. (Cl. 75-82) The present invention relates to the refining of alloys to recover valuable constituents thereof. More particularly, the present invention relates to the processing of nickel base and cobalt base alloys to eliminate minor alloying constituents therefrom without significant loss of nickel or cobalt.

A wide variety of so-called high-temperature resistant alloys have been developed for use in gas turbines, exhaust gas-driven turbines and compressors, and similar devices where high tensile strength and minimum creep are required at temperatures on the order of 1500 to 1800" F. These alloys are generally based on nickel or cobalt or combinations of the two metals strengthened by additions of other metals such as chromium, molybdenum, tungsten, columbium, and may contain various amounts of iron. Some of these alloys are strengthened by precipitation hardening, the precipitate being formed either from two or more of the above-named elements or from additional elements such as titanium, aluminum, boron, and the like. In addition, such alloys usually contain carbon, silicon, and manganese.

In spite of the excellent properties of these alloys, service life is limited and consequently, considerable quantities of used metal have been accumulated. Because of the high economic value of the base metals contained in these alloys, there is considerable inducement to develop means for converting the scrap to useful form.

The aforementioned type of alloys most often cannot be reused without prior refining due to the unpredictable and uncontrollable recovery of some of the constituents, and also because some of the minor alloy constituents are not desired in all the products that might be made from this source of metal.

In general, it is desired to recover the maximum possible amount of the nickel, cobalt, chromium and molybdenum and eliminate or materially reduce the content of the remaining elements. At times, however, it is desired to also eliminate chromium and molybdenum from nickel and cobalt base alloys.

One approach to the problem has been to attempt selective oxidation of the undesired elements by blowing the molten metal with oxygen. This resulted in almost complete oxidation and removal of tungsten, columbium, molybdenum, and chromium. About half of the iron was removed by this procedure which is not always desirable. This work was reported by the US. Bureau of Mines in R1. 5786. An attempt to use this process to refine a nickel-cobalt-chromium-molybdenum alloy resulted in excessive loss by oxidation of chromium and molybdenum. Also, the operating costs of this technique are not always favorable.

It is therefore an object of the present invention to provide a relatively simple and economical process for recovering nickel, cobalt and other valuable constituents from nickel and cobalt base alloys.

Other objects will be apparent from the following description and claims.

It has been discovered, as a part of the present invention, that by preparing a fused mixture of the alloy to be refined with basic slag and a selected metal oxide, e.g. chromium oxide, and reacting the selected metal oxide with the alloy, that alloy constituents such as titanium,

aluminum, silicon, vanadium, zirconium, columbium, tantalum, boron and carbon can be eliminated from the alloy.

It has also been discovered that chromium, in addition to the aforementioned alloy constituents, can also be removed from nickel and cobalt base alloys when iron oxide is used as the selected metal oxide.

It has further been discovered that molybdenum can also be removed from nickel and cobalt base alloys, in addition to chromium and the other specified alloy constituents, by using nickel oxide as the selected metal oxide.

In the practice of the present invention, a basic slag is melted in a refractory lined arc furnace and a mixture of the alloy to be refined and the selected metal oxide is added in increments and the resultant fused mixture is heated until evidence of reaction has ceased. In the course of the reaction, the undesired alloy constituents are oxidized by reaction with the selected metal oxide and go into the slag. The slag is then decanted and the refined metal is cast into suitable molds.

In the present invention, the slag together with the selected metal oxide addition should be basic and the amount of the slag should be approximately 0.80 to 1.25 by weight of the weight of the final alloy product. Slags which are particularly suitable for the practice of the present invention are those derived as by-products of aluminum reduction operations. These slags usually contain CaO, A1 0 and MgO in about the following proportions:

CaO 25 to 50 A1 0 40 to 70 MgO 1 to 15 The amount of selected metal oxide used in the practice of the present invention should contain at least the stoichiometric amount of oxygen necessary to c mplet ly oxidize the alloy constituents which are to be removed. Preferably, an excess of selected metal oxide is used containing on the order of 1.5 to 4 times the amount of oxygen necessary to oxidize the undesired alloy constituents.

In the present invention, when refining a chromium containing nickel or cobalt base alloy, and it is desired to retain the chromium in the alloy, chromium oxide, Cr O is used as the selected metal oxide. With the use of chromium oxide in the manner of the present invention, alloy constituents such as titanium, vanadium, columbium, tantalum, boron, carbon, aluminum and silicon can be removed from the alloy. Cobalt, nickel, molybdenum and tungsten are substantially unalfected and are recovered in the refined alloy product.

Chromium oxide can also be used as the selected metal oxide in treating a chromium-free alloy when a chromium content is acceptable in the refined alloy,

When it is desired to eliminate chromium from nickel or cobalt base alloys, in addition to the previously mentioned alloy constituents, viz Ti, V, Cb, Ta, B, C, Al, Si, or when a chromium content is not desired in the refined alloy, iron oxide is used as the selected metal oxide.

Nickel oxide can be used as the selected metal oxide, with or in place of iron oxide, when it is desired to provide a molybdenum depleted alloy. That is, nickel oxide can be used to remove chromium and molybdenum, in addition to the previously mentioned minor alloy constituents which are removed through the use of iron oxide.

With regard to any iron and manganese constituents in the treated alloy, it has been found that the recovery of these materials varies with the basicity of the slag used, the recoveries increasing with increasing basicity of the slag.

The following examples are provided to further illusirate the refinement of nickel and cobalt base alloys in ac- .cordance with the present invention. For purposes of the present invention, nickel base and cobalt base alloys are those in which the sum of the nickel and cobalt contents represents a predominant proportion of the alloy.

The composition of the starting and final materials is set forth below:

Starting mixture Pounds EXAMPLE I NiCoCr--Mo alloy 150 Six hundred pounds of CaO-Al O MgO slag were i (1(Fe3O4) ag melted in a refractory lined arc furnace. A mixture of me e OX1 e 25 the metal to be refined and chromium oxide was added 3 9 25 incrementally to the molten slag and the resultant molten 1O 1 lca San mixture was heated at a temperature of about 17001800 Mill scale C. until all evidence of reaction ceased which took about Percent one hour. The total amount of metal added was 1000 Fe 73.61 lbs. and the total amount of chromium oxide added was SiO 0.24 300 lbs. MnO 0.74 After cessation of reaction in the molten mixture, C 0.17 the slag was decanted and the refined metal was cast into B lack nickel oxide P 1g molds Percent The slag and metal composltions are set forth below: Ni 7618 Starting slag (including Cr O addition) C0 0.02 Fe 0.60 Percent Si 0 09 C210 345 A1 0 26.4 M O 2 1 Starting Metal Percent Refined Metal Percent g 5 (150 lbs.) (150 l Cr 0 1 34.9 SiO TiO etc. Balance 4182 1 Specification of chromium oxide used 2' C1 percent, miu 67 S percent, max 0.2 20 C 0 0.05 L51 Fe d0 0.01 0.01 Loss on ignition do 0.5 03 1.90

Original Alloy Refined Alloy Composition Composition (Weight 1,000 Weight; Final slag lbs.), Percent Percent Percent Cr 9.52 54.30 53.86 17.80 20.74 M0 :22 Fe 29.88 1: 17 :3 Ni 4.09 0.04 0. 03 What IS claimed is: 3 8-5 1. A process for refining nickel base and cobalt base 0: 16 0: 05s alloys by removing minor alloying constituents therefrom 8'812 41 including titanium, aluminum and silicon, said process 0'. 07 comprising providing a fused mixture of the alloy to be 3:82 treated with a basic slag and chromium oxide; and reacting said chromium oxide with the admixed alloy to cause *735 lbs. were recovered as cast pigs; the skull left in the furnace (which can be recovered) was not weighed.

Final slag Percent CaO 31.6 A1 0 32.4 C1203 MgO 7.3 SiO- 0.9 TiO 2.1

The product obtained was eminently suited for reuse in the preparation of other alloys containing nickel, cobalt, chromium, and molybdenum.

EXAMPLE II oxidation and removal of minor alloy constituents including titanium, aluminum and silicon.

2. A process for refining nickel base and cobalt base .alloys which contain minor proportions of at least one metal selected from the group consisting of titanium and silicon, said process comprising providing a fused mixture of the alloy to be treated with a basic slag and chromium oxide; and reacting said chromium oxide with the admixed alloy to cause oxidation and removal of said selected metal.

References Cited by the Examiner UNITED STATES PATENTS 1,218,412 3/ 1917 Kissock 84 1,552,609 9/ 1925 Hybinette 75--82 1,674,438 6/ 1928 Hybinette 75-82 2,049,721 8/ 1936 Perrin 75-82 2,902,359 9/1959 SChOtt 75-82 3,030,201 4/ 1962 Queneau 75--82 FOREIGN PATENTS 117,719 11/ 1943 Australia.

HYLAND BIZOT, Primary Examiner,

BENJAMIN HENKIN, Examiner. 

1. A PROCESS FOR REFINING NICKEL BASE AND COBALT BASE ALLOYS BY REMOVING MINOR ALLOYING CONSTITUENTS THEREFROM INCLUDING TITANIUM, ALUMINUM AND SILICON, SAID PROCESS COMPRISING PROVIDING A FUSED MIXTURE OF THE ALLOY TO BE TREATED WITH A BASIC SLAG AND CHROMIUM OXIDE; AND REACTING SAID CHROMIUM OXIDE WITH THE ADMIXED ALLOY TO CAUSE OXIDATION AND REMOVAL OF MINOR ALLOY CONSTITUENTS INCLUDING TITANIUM, ALUMINUM AND SILICON. 